如何实现递归泛型的类型类？

Question

I would like to use type classes to provide a specialized implementation for a generic class. The trouble is the class is recursive, and I did not find a way how to write the code so that is works. Here is my attempt:

object Compare {
  trait Compare[T <: Compare[T]] {
    def compare(that: T): Boolean
  }

  trait IsCompare[T] {
    def check: Boolean
  }

  trait LowerLevelImplicits {
    implicit def defaultCompare[T]: IsCompare[T] = new IsCompare[T] {
      def check = false
    }
  }

  trait Implicits extends LowerLevelImplicits {
    implicit def isCompare[T: Compare]: IsCompare[T] = new IsCompare[T] {
      def check = true
    }
  }

  case class MyClass(value: Int) extends Compare[MyClass] {
    override def compare(that: MyClass) = this equals that
  }
}

import Compare._

object Main extends App with Implicits {

  def matchArray[T: IsCompare](array: Array[T]) = {
    if (implicitly[IsCompare[T]].check) {
      println("Using isCompare")
    } else {
      println("Not using isCompare")
    }
  }


}

The error is:

Error:(17, 29) type arguments [T] do not conform to trait Compare's type parameter bounds [T <: Compare.Compare[T]]

 implicit def isCompare[T: Compare]: IsCompare[T] = new IsCompare[T] { 

Answer 1

尝试将isCompare的方法签名更改为：

implicit def isCompare[T <: Compare[T]]: IsCompare[T] = //...